The present conventional method of manufacturing integrated circuits involves photolithographic replication of patterns from a mask onto a silicon wafer surface, which has been treated with a photoresist material. Each such step is followed by conventional procedures such as developing, etching, etc. These steps may be repeated a number of times on a single wafer, each pattern overlying patterns previously applied. With increasing miniaturization, the pattern elements have become smaller and smaller to the extent that pattern resolution is limited by the wavelength of light. This has already led to the use of the shorter wavelengths of the ultraviolet spectrum. The successor technology appears to be x-ray lithography.
One of the problems which will arise in the use of x-ray lithography is that the procedures for aligning the masks and wafers are intricate and time consuming.
One system for aligning an opaque mask with an integrated circuit wafer is described in U.S. Pat. application Ser. No. 149,106 filed May 12, 1980 and now U.S. Pat. No. 4,335,313, wherein an opaque lithographic mask is aligned to a substrate wafer by providing three alignment targets on one surface of each of the mask and the wafer. Three microscope optical channels are positioned to view portions of a reference plane. The wafer is positioned with its alignment targets in the reference plane. The optical axis of each of the optical channels is aligned with a different one of the three targets on the wafer. The wafer is then displaced from the reference plane to an exposure plane parallel to the reference plane, while retaining its alignment relative to the optical axes. This displacement provides a known mask to wafer gap. The mask is placed in the reference plane and is positioned to align each of its three alignment targets with one of the optical axes without changing the alignment of the axes. The mask and wafer are then transported to an exposure station while retaining the alignment therebetween.
It is an object of the present invention to provide a new and improved apparatus for effecting alignment and spacing control between a mask and wafer for use in x-ray lithography, which overcomes or at least mitigates a number of the problems encountered by such prior art apparatus.